1. Field of the Invention
The present invention relates to an optical focus position control device of an optical disc apparatus that records, plays back, and erases a variety of information by irradiating optical beams such as the laser beams onto a recording media. The present invention relates, more particularly, to an optical focus position control device in an opto-magnetic disc apparatus which records, plays back, and erases a variety of information by irradiating optical beams such as laser beams onto a recording medium including a magnetic film.
2. Prior Art
Conventionally, any of the existing optical discs have surfaces which easily vibrate during rotation, and as a result, recording tracks on the disc are displaced in the direction of the optical axis of the incident laser beams that irradiate the disc surface. Also, the recording tracks are adversely affected by any deviation between the center position of the disc and the motor shaft that drives the disc. Recording tracks of such a disc are then displaced in the direction of the disc radius (hereinafter called the radial direction). To prevent any displacement of recording tracks, a device is provided so that the laser beam focus position can be correctly corrected within the optical head mechanism to enable the incident laser beam spot to correctly match the recording tracks of a disc. Such a device is called the optical focus position control in the following description.
Using any of the existing optical discs such as those that only plays back information without containing any magnetic film recording media, or those that can record any additional information, fine adjust the focus position of the incident laser beams (hereinafter called the focus controller) must be accomplished to deal with the disc displacement in the direction of the optical axis of the incident laser beams. A device that can vary the position of the objective lens by means of the electromagnetism is well known by now. On the other hand, to fine adjust the focus position of the incident laser beams (hereinafter called the tracking controller) to deal with the disc displacement in the radial direction, a variety of mechanisms for this fine adjustment of the focus position of the incident laser beams have been introduced which utilize a rotary mirror that reflects the incident laser beams in any optimum direction. The above-mentioned tracking control is not practical because the incident laser beams are inevitably inclined from the perpendicular direction of the disc.
Accordingly, a new proposal has been introduced quite recently, which provides a mechanism capable of jointly performing both the focus and tracking controls mentioned above, by varying the position of the objective lens using electromagnetic force. Basically, such a mechanism comprises a coil that can be moved integrally with an objective lens and a stationary permanent magnet, thus causing the objective lens to be displaced by the current flowing through said coil.
However, this mechanism provides the following problem.
For Example, a problem arises when a mechanism incorporating a cylindrical objective lens support which is supported by rubber material whose one end is secured to a stationary holder, where said cylindrical objective lens support is driven by electromagnetic force existing between the coil secured to said cylindrical objective lens support and the magnetic circuit secured to said stationary cylinder. Since said rubber-elastic material supports the cylindrical objective lens support it cannot fully resist the tilting force of the cylindrical objective lens support, and as a result, any accidental force may be generated when the drive force generated by said electromagnetic force cannot be applied to the gravity center of the cylindrical objective lens support, thus causing said cylinder to eventually generate a rotary movement. This will cause the optical axis of beams to tilt against the center axis of the objective lens, and so either the off-axis astigmation or coma aberration will adversely affect the disc tracks containing information, causing beams to poorly focus on them, and as a result, quality of the recorded information will be degraded significantly.
Furthermore, if such an already known mechanism capable of jointly performing both the focus and tracking controls by varying the position of the objective lens via the electromagnetic force is actually applied to an optical disc apparatus, it will easily create problems described below.
Since the proposed mechanism uses magnetism generated by a permanent magnet, a leakage magnetism or flux will be generated in portions peripheral to the disc. Nevertheless, since the disc uses the magnetic film for the recording media, if such a leakage mechanism adversely affects said magnetic film, the following problems will arise.
(1) When the laser beams are irradiated onto an optical magnetic disc to cause temperature to rise, and simultaneously an information is recorded on said disc via an external magnetism, if a leakage magnetism out from said optical focus position control affects the disc, then the quality of the recorded information will be degraded significantly.
(2) When playing back the recorded information via the magnetic-optical effect by irradiating laser beams onto the optical magnetic disc, any leakage flux from said optical focus position control may adversely affect the disc, thus causing the recorded information to be easily erased.
In the light of these potential disadvantages, if an optical disc apparatus is used, it is quite necessary to completely prevent even the slightest leakage flux from the optical focus position control from seriously affecting the optical disc. In addition, there are still further problems to solve.